Process for the recovery of sulphur dioxide and ammonia



United States Patent 3,383,170 PROCESS FOR THE RECOVERY OF SULPHUR DIOXIDE AND AMMONIA Herbert Furkert, Junkersdorf, Cologne, and Hans Muehlenbein, Cologne-Lindenthal, Germany, assignors, by mesne assignments, to Chemiebau Dr. A. Zieren GrnbH & Co. KG, Cologne-Braunsfeld, Germany, a corporation of Germany No Drawing. Filed Jan. 12, 1965, Ser. No. 425,054

Claims priority, application9(.l6ermany, Jan. 17, 1964,

The portion of the term of the patent subsequent to May 23, 1984, has been disclaimer! and dedicated to the Public 12 Claims. (Cl. 23-177) ABSTRACT OF THE DISCLOSURE A process for the recovery of ammonia and sulfur dioxide from feeds containing (NH SO and/or NH HSO When the feed is rich in (NH SO an addition containing a major portion of (NH SO is made. The mixture is then heated to decompose the (NH SO to form NH HSO and ammonia. The ammonia is recovered. The NH HSO is partially reduced by heating with a non-carbon reducing agent at a temperature above its melting point, to form S0 and (NH SO The resultant mixture is recycled to the feed. When the feed is rich in NH HSO an addition containing a major portion of NH -HSO is made to the feed. This mixture is then partially reduced with a non-carbon containing reducing agent at a temperature above the melting point of the NH HSO to liberate S0 and to form (NH SO The resultant mixture of (NH SO and unreduced NH4HSO, is recycled and admixed with the feed. The process is controlled so that the ammonia produced in the decomposition step, and the S0 produced in the reducing step, correspond to the initial ammonia and sulfate content of the feed.

The present invention relates to a process for recovering sulfur dioxide and ammonia from the ammonium salts of sulfuric acid.

Such ammonium salts are often produced as by-products from various industrial processes, including recovery of spent acids from petroleum chemistry, from titanium dioxide production, recovery of ammonium salts from the nitrile saponification with sulfuric acid, and from processes where ammonia is scrubbed with sulfuric acid. Although the literature describes various processes for working up ammonium sulfate and ammonium hydrogen sulfate respectively, such processes involve economic or technical difiiculties when applied on plant scale.

In A. M. Schtschuharew, C36I667, the (NH SO is heated with Fe O whereby first the N H and then by calcination the S0 is set free. I. W. Delapaine, Chem. En'gng. Progn, 51, 499-503 (1955) uses zinc oxide instead of R 0 In ER 2118 it is suggested that (NH SO be heated together with Na SO whereby upon formation of NaHSO the NH;, escapes. The sodium hydrogen sulphate then releases S0 at higher temperatures. The processes referred to have not attained technical feasibility for use in the field.

It is one object of the present invention to provide an economic process for recovering ammonia and sulfur dioxide from feeds containing ammonium sulfate and/or ammonium hydrogen sulfate.

It was now found out that ammonium sulphate or hydrogen sulphate or a mixture of both salts respectively can easily be decomposed into NH;, and S0 if, instead of the above-mentioned admixtures as ZnO, or Na SO ice a (NH SO quantity being equivalent to these materials is added to the chemical cycle, then expelling first the NH from the (NH SO by heating to 200-450 (3., preferably at 400 C., and finally converting the NH HSO with a reducing agent, upon separation of S0 into ammonium sulphate again which is kept in the cycle. It could be stated that the use of reducing agents containing C (already described in the literature) cause substantial ammonia losses and that H H 5, or sulphur surprisingly reduce so carefully, that the NH, yield is at least The process procedure is illustrated by the following formulae.

(a) Ammonium sulphate is treated:

400 (NHlMSOK Ste) (NH4)2SO4(cycle) 2NH: 2NH4HSO;

(NH4)zSO4 -r 2NH3 S0: E20 0- (b) Ammonium hydrogen sulphate is treated:

(III) red. NHAHSOJ (waste) N HiHSOr (cycle) (NHDZSOQ S02 1120 0- (NHQQSO; NH NH4HSO4 (cycle) Expelling of the NH by the Formula I or IV respectively may be accelerated by introducing inert gas or steam or by vacuum. It is not necessary to convert all (NH SO into NH HSO since release of the last percent NH is not economical. Heating of the solid is effected advantageously by introducing it into a stock of salt already heated to 35 0400 C. and molten. A quantity equal to the feed is permanently withdrawn from this stock and introduced into a column where it is treated in counter-current with superheated steam or other hot inert gases to decompose most of the (N-H SO into NH HSO and ammonia which is recovered. The ammonium hydrogen sulphate or pyro sulphate respectively, flowing off the column, is reduced at temperatures between 4-00 and 550 C., i.e., liquid or also in the form of vapors, to such an extent that the (NH SO salt is produced according to Equation II when cooled to less than 320 C. The reduction of the pyro sulphate can slightly be accelerated by steam. The procedure as per Equation II or III, respectively, is to be understood that of the 2 moles NH HSO one mole is reduced to NH HSO or NH S0 and H 0, respectively, which at lower temperature is reacted with the second mole of NH HSO from the neutral salt and S0 The S0 obtained undergoes, if it still contains sulphur scraps from reduction, a recombustion and is then best delivered to a sulphuric acid plant.

Depending on the composition of the ammonium sulphates to be decomposed, the place where same are added to the cycle changes. Neutral sulphate or a mixture with a minor quantity of acid sulphate is best added directly to the NH expulsion step (1). Acid sulphate or mixtures with minor quantites of neutral salt need first be converted into the neutral salt by reduction (step 2).

From the foregoing it is apparent that the invention provides an economical cyclic process for recovering ammonia and sulfur dioxide from feed stocks containing ammonium sulfate and/or'ammonium hydrogen sulfate. The cyclic process basically consists of two operations, denoted steps herein. Step (1) involves decomposing ammonium sulfate at a temperature between about 200 C. and 450 C, and preferably at about 400 C. to form ammonia and ammonium hydrogen sulfate. When the feed to the cyclic process is largely or totally composed of ammonium sulfate, the feed is introduced into the cyclic process at or just before step (1). An amount of ammonium sulfate, preferably equal to the amount of ammonium sulfate in the feed, which has been recycled from a later step in the cyclic process is mixed with the feed. Substantially all of the ammonium sulfate is decomposed in the first step of the process. Since it is not economic to carry the yield to 100% completion, the product of the first step of the process will contain a small amount of ammonium sulfate admixed with the ammonium hydrogen sulfate. This step of the process is preferably carried out by contacting the ammonium sulfate at the elevated temperature with superheated steam in countercurrent relation.

In a second step of the process, the ammonium hydrogen sulfate product of the first step is partially reduced to recover S When the feed to the cyclic process is largely or totally composed of ammonium hydrogen sulfate, the feed is introduced into the cyclic process at or just prior to step (2). When an ammonium hydrogen sulfate feed is used, the amount thereof is preferably controlled so that it is equal to the amount of ammonium hydrogen sulfate product of the decomposition step (1) of the process. Of the total ammonium hydrogen sulfate treated in the second step of the process, the process is controlled so that substantially one-half of the ammonium hydrogen sulfate is reduced to NH HSO (equivalent to NH S0 and H 0, as set forth in Equation II hereinbefore). This is preferably carried out by diverting substantially one-half of the NH HSO around the reduction operation and then combining this diverted NH HSO with the product of the reduction operation to form (NI- Q 50 which is recycled and S0 which is recovered.

By controlling the process so that the amount of feed is substantially equal to the corresponding salt being cycled from step (1) to step (2), or from step (2) to step (1), respectively; and by reducing substantially one-half of the NH HSO in step (2), the process is controlled to produce NH and S0 in amounts substantially corresponding to the NH; and sulfate content in the feed.

The invention is further llustrated in the following examples:

Example 1 For decomposition of 66 kgs. spent (NH SO into NH;, and S0 132 kgs. ammonium sulphate are delivered within ten minutes to a receiver which contains about 111 kgs. of an (NH SO melt heated to 380-400 C., whereby water and part of the ammonia escape. The reaction procedure is adjusted so that a sufiicient quantity of heat is delivered to maintain the temperature between 380 C. and 400 C. A quantity of (NH SO melt equivalent to the feed is withdrawn continuously at the bottom of the vessel. The ammonium hydrogen sulphate and ammonium sulphate mixture flowing off is delivered to a packed column where the residual ammonia is expelled at 400 C. by superheated steam in counter-current flow. The escaping mixture of steam and about 17 kgs. NH is condensed. The NH HSO flowing off the column is evaporated by means of a hot S0 and N gas mixture produced by the combustion of sulphur. One-half of said NH SO is reduced at about 440 in direct current with 0.8 kg. sulphur vapor within 20 minutes. When cooling to at least 300 C., 64 kgs. ammonia sulphate and the S0 (32 kgs.) equivalent to the sulphate sulphur are obtained. The (NH SO returns together with new ammonium sulphate to the chemicals cycle.

The sulphur being contained in the SO; and not con- 4 verted undergoes re-combustion and the S0 resulting therefrom is preferably delivered to a sulphuric acid plant.

Example 2 One-half of the NH HSO flowing off the packed column as per Example 1, is evaporated and then reduced completely to NH HSO which is at this temperature dissociated to S0 NH and H 0. This gas mixture is after combustion of the sulphur not being converted, cooled to about C. and then converted in a spray dryer under injection of the remaining half of the NH I-ISO into (NHQ SO The (NI-I SO obtained is returned again to the chemicals cycle.

Example 3 In a continuous plant, 10 kgs./min. of NH HSO are circulated. 5 kgs. of this amount are branched off onto a furnace, in which they are reduced by means of 696 g./ min. of sulfur to NH HSO The required reaction temperature of about 450 C. is produced by burning of additional sulfur with air. At first the NH HSO is washed out from the escaping reaction products with an aqueous ammonia-sulfate solution or with water and then the remaining half of the original NH HSO is fed to this solution. The S0 is boiled off. The ammonium salt precipitated after evaporation of the water is fed to the plant in order to separate the ammonia. The ammonia which is becoming free is suitably dissolved in water. The losses are 5 The obtained S0 can be liquified, reduced to sulfur or can be fed to a sulfuric acid plant.

As many embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that the present invention includes all such modifications and variations as come within the scope of the appended claims.

We claim:

1. A cyclic process for the recovery of NH and S0 from a (NI-I SO rich feed comprising:

(1) adding (NH SO to said feed and heating to a temperature between 200 C. and 450 C. to decompose said (NH SO and to form NH HSO and NH recovering said NH (2) heating said NH HSO with a non-carbon containing reducing agent at a temperature above the melting point of said NH HSO whereby a portion of said NH HSO is reduced to NH HSO and is reacted with the unreduced NH HSO' to liberate S0 which is recovered, and to form (NH SO and (3) recycling said (NH.,) SO to step (1);

wherein the process is controlled so that the NH produced in said decomposition step (1) and the S0 produced in said reducing step (2) corespond to the initial NH, and sulfate content of said feed.

2. The cyclic process, as set forth in claim 1, wherein said feed is selected from the group consisting of and a mixture of a major amount of (NH SO and a minor amount of NH HSO 3. A cyclic process for the recovery of NH;, and SO; from a (NHQ SO. rich feed comprising (1) adding (NH SO to said feed and heating wit steam at a temperature of between 200 C. and 450 C. to decompose said (NH SO and to form NH; and NH HSO recovering said NH and (2) (A) heating, said NH HSO at about 450 C. with a reducing agent selected from the group consisting of H S, and H 5, to form NH HSO and (B) reacting said NH HSO with an equal weight of and (3) recycling said (NH SO to step (1);

wherein the process is controlled so that the NH produced in said decomposition step (1) and the S0 produced in said reducing step (2) correspond to the initial NH, and sulfate content of the feed salt.

4. The process, as set forth in claim 3, wherein said (NH SO is heated with steam at a temperature of about 400 C.

5. The cyclic process, as set forth in claim 4, wherein said feed is selected from the group consisting of (NH SO and a mixture of a major amount of (NH SO and a minor amount of NH HSO 6. A cyclic process for the recovery of NH and S0 from a feed rich in NH HSO comprising:

(1) adding NH HSO to said feed and heating with a non-carbon containing reducing agent at a temperature above the melting point of said NH HSO whereby a portion of said NH HSO is reduced to NH HSO and is reacted with the unreduced to liberate S0 which is recovered, and to form and (2) heating said (NH SO to a temperature between 200 C. and 450 C. to decompose said (NI-I SO and to form NH HSO and NH which is recovered and (3) recycling said NH I-ISO to step (1);

wherein the process is controlled so that the NH produced in said decomposition step (2) and the S0 produced in said reducing step (1) corresponds to the initial NH; and the sulfate content of said feed.

7. The process as set forth in claim 6 wherein said (NI-1.9 80 is heated with steam at a temperature of about 400 C.

8. The cyclic process, as set forth in claim 6, wherein said feed is selected from the group consisting of NH HSO and a mixture of a major amount of NH HSO and a minor amount of (NH SO 9. The cyclic process, as set forth in claim 7, wherein said feed is selected from the group consisting of NH HSO and a mixture of a major amount of and a minor amount of (NH SO 10. A cyclic process for the recovery of NH and S0 from a feed rich in NH HSO comprising:

(1) (A) adding NI-I HSO to said feed and heating at about 450 C. with a reducing agent selected from the group consisting of H S, and H S, to form NH HSO and (B) reacting said NH HSO with an equal weight of NH HSO, to form S0 which is recovered, and to form (NH SO and (2) heating said (NH SO with steam to a temperature between 200 C. and 450 C. to decompose said (NH SO and to form NH HSO and NH which is recovered; and

(3) recycling said NH HSO to step (1);

wherein the process is controlled so that the NH; produced in said decomposition step (2) and the S0 produced in said reducing step (1) corresponds to the initial NH; and the sulfate content of said feed. '1'

11. A cyclic process for the recovery of NH and S0 from a feed rich in (NH SO comprising:

(1) admixing said feed with a substantially equal amount of recycled (NH SO and passing said (NH SO counter-clurrent to superheated steam at a temperature of about 400 C. tofidecompose said (NI-1.0 80 into NH and NH HSO recovering said NH and (2) introducing substantially one-half of said into a chamber at a temperature of about 450 C., wherein sulfur is added to reduce said NH HSO into NH HSO and then combining said NH HSO with the remaining one-half of said NH HSO to form :SO which is recovered, and (NI-1.9 80 and contacting said (NH SO with steam at a temperature of about 400 C. and countercurrent to said steam, to decompose said (NI-19 50 into NHg which is recovered, and into Nl-I HSO' which is then admixed with said :feed.

References Cited UNITED STATES PATENTS 5/1967 Furkert et al. 23178 8/1946 Hixson et a1. 23-178 OTHER REFERENCES Mellor: Comprehensive Treatise on Inorganic and Theoretical Chemistry, vol. 2, p. 705 (1922).

OSCAR R. VERTIZ, Primary Examiner. BENJAMIN HENKIN, Examiner.

R. M. DAVIDSON, A. J. GREIF, Assistant Examiners. 

